This invention relates generally to vaccines and more particularly to the production of oral vaccines in edible transgenic plants and the administration of the oral vaccines such as through the consumption of the edible transgenic plants by humans and animals.
Diseases have been a plague on civilization for thousands of years, affecting not only man but animals. In economically advanced countries of the world, diseases are 1) temporarily disabling; 2) permanently disabling or crippling; or 3) fatal. In the lesser developed countries, diseases tend to fall into the latter two categories, permanently disabling or crippling and fatal, due to many factors, including a lack of preventative immunization and curative medicine.
Vaccines are administered to humans and animals to induce immunity via antibody production or cell-mediated immunity against viruses, bacteria, and other types of pathogenic organisms. In the economically advanced countries of the world, vaccines have brought many diseases under control. In particular, many vital diseases are now prevented due to the development of immunization programs. The virtual disappearance of smallpox, certainly, is an example of the effectiveness of a vaccine worldwide. But many vaccines for such diseases as poliomyelitis, measles, mumps, rabies, foot and mouth, and hepatitis B are still too expensive for the lesser developed countries to provide to their large human and animal populations. Lack of these preventative measures for animal populations can worsen the human condition by creating food shortages.
The lesser developed countries do not have the monetary funds to immunize their populations with currently available vaccines. Not only the cost of producing the vaccine is limiting, but the further cost of the professional administration of the vaccine is prohibitive in many cases, especially for vaccines that require multiple doses to maintain immunity. Therefore, often, the countries that need the vaccines the most can afford them the least.
Underlying the development of traditional vaccines is the ability to grow the disease causing agent in large quantities. At the present, most vaccines are produced from killed or live attenuated pathogens. If the pathogen is a virus, large amounts of the virus must be grown in an animal host or cultured animal cells. If a live attenuated virus is utilized, it must be clearly proven to lack virulence while retaining the ability to induce humoral and cellular immunity. If a killed virus is utilized, the vaccine must demonstrate the capacity of surviving antigens to induce immunization. Additionally, surface antigens, the major viral particles which induce immunity, may be isolated and administered to proffer immunity in lieu of utilizing live attenuated or killed viruses.
Vaccine manufacturers often employ complex technology entailing high costs for both the development and production of the vaccine. Concentration and purification of the vaccine is required, whether it is made from the whole bacteria, virus, other pathogenic organism or a sub-unit thereof. The high cost of purifying a vaccine in accordance with Food and Drug Administration (FDA) regulations makes oral vaccines prohibitively expensive to produce because they require ten to fifty times more than the regular quantity of vaccine per dose than a vaccine which is parenterally administered. Of all the viral vaccines being produced today only a few are being produced as oral vaccines.
According to FDA guidelines, efficacy of vaccines for humans must be demonstrated in animals by demonstrating evidence for humoral and cellular immunity and by resistance to infection and disease upon challenge with the pathogen. When the safety and immunogenicity levels are satisfactory, FDA clinical studies are then conducted in humans. A small carefully controlled group of volunteers are enlisted from the general population to begin human trials. This begins the long and expensive process of testing. It may take years before it can be determined whether the candidate vaccine can be given to the general population. If the trials are successful, the vaccine may then be mass produced and sold to the public.
Even after these precautions are taken, problems can arise. With the killed virus vaccines, there is always a chance that one of the live viruses has survived and vaccination may lead to isolated cases of the disease. Moreover, since both the killed and live attenuated types of virus vaccines are made from viruses grown in animal host cells, the vaccines are sometimes contaminated with cellular material from the animal host which can cause adverse, sometimes fatal, reactions in the vaccine recipient. Legal liability of the vaccine manufacturer for those who are harmed by a rare adverse reaction to a new or improved vaccine necessitates expensive insurance which ultimately adds to the cost of the vaccine.
Some vaccines have other disadvantages. Single inoculations of vaccines prepared from whole killed virus generally stimulate the development of circulating antibodies (IgM, IgG) thereby conferring a limited degree of immunity which usually requires boosting through the administration of additional doses of vaccine at specific time intervals. Live attenuated viral vaccines, while much more effective, have limited shelf-life and storage problems requiring maintaining vaccine refrigeration during delivery to the field..sup.1
Efforts today are being made to produce less expensive vaccines which can be administered in a less costly manner. Recombinants or mutants can be produced that serve in place of live virus vaccines. The development of specific deletion mutants that alter the virus, but do not inactivate it, yield vaccines that can replicate but cannot revert to virulence.
Recombinant DNA techniques are being developed to insert the gene coding for the immunizing protein of one virus into the genome of a second, avirulent virus type that can be administered as the vaccine. Recombinant vaccines may be prepared by means of a vector virus such as vaccinia virus or by other methods of gene splicing. Vectors may include not only avirulent viruses but bacteria as well. A live recombinant hepatitis A vaccine has been constructed using attenuated Salmonella typhimurium as the delivery vector via oral administration..sup.1
Various avirulent viruses have been used as vectors. The gene for hepatitis B surface antigen (HBsAg) has been introduced into a gene non-essential for vaccinia replication. The resulting recombinant virus has elicited an immune response to the hepatitis B virus in test animals. Additionally, researchers have used attenuated bacterial cells for expressing hepatitis B antigen for oral immunization. Importantly, when whole cell attenuated Salmonella expressing recombinant hepatitis antigen were fed to mice, anti-viral T and B cell immune responses were observed. These responses were generated after a single oral immunization with the bacterial cells resulting in high-titers of the immune responsive cells. See, e.g., "Expression of hepatitis B virus antigens in attenuated Salmonella for oral immunization," F. Schodel and H. Will, Res. Microbiol., 141:831-837 (1990). Others have had similar success with oral administration routes for recombinant hepatitis antigens. See, e.g., M. D. Lubeck et al., "Immunogenicity and efficiacy testing in chimpanzees of an oral hepatitis B vaccine based on live recombinant adenovirus," Proc. Natl. Acad. Sci. 86:6763-6767 (1989); S. Kuriyama, et al., "Enhancing effects of oral adjuvants on anti-HBs responses induced by hepatitis B vaccine," Clin. Exp. lmmunol. 72:383-389 (1988); J. C. Nedrud and N. Sigmund, "Cholera Toxin as a Mucosal Adjuvant: III. Antibody responses to nontarget dietary antigens are not increased," Reg.--Immunol. 3:217-222 (1991); J. D. Clements, et al., "Adjuvant activity of Escherichia coli heat-labile enterotoxin and effect on the induction of oral tolerance in mice to unrelated protein antigens," Vaccine 6:269-277 (1988); K. S. Chen and W. Strober, "Cholera holotoxin and its B subunit enhance Peyer's patch B cell responses induced by orally administered influenza virus: disproportionate cholera toxin enhancement of the IgA B cell response," Eur. J. Immunol. 20:433-436 (1990).
Other virus vectors may possess large genomes, e.g. the herpesvirus. The oral adenovirus vaccine has been modified so that it carries the HBsAg immunizing gene of the hepatitis B virus. Chimeric polio virus vaccines have been constructed of which the completely avirulent type 1 virus acts as a vector for the gene carrying the immunizing VP1 gene of type 3..sup.1
Immunity to a pathogenic infection is based on the development of an immune response to specific antigens located on the surface of a pathogenic organism. For enveloped viruses, the important antigens are the surface glycoproteins. Glycosylation of viral surface glycoproteins is not always essential for antigenicity..sup.1 Unglycosylated herpesvirus proteins synthesized in bacteria have been shown to produce neutralizing antibodies in test animals..sup.1 However, where recombinant antigens such as HBsAg are produced in organisms requiting complex fermentative processes and machinery, the costs and access can be prohibitive.
Viral genes which code for a specific surface antigen that produces immunity in humans or animals, can be cloned into plasmids. The cloned DNA can then be expressed in prokaryotic or eukaryotic cells if appropriately engineered constructions are used. The immunizing antigens of hepatitis B virus, .sup.2 foot and mouth, .sup.3 rabies virus, herpes simplex virus, and the influenza virus have been successfully synthesized in bacteria or yeast cells..sup.1
Animal and human subjects infected by a pathogen mount an immune response when overcoming the invading microorganism. They do so by initiating at least one of three branches of the immune system: mucosal, humoral or cellular. Mucosal immunity largely results from the production of secretory IgA antibodies in the secretions that bathe mucosal surfaces in the respiratory tract, the gastrointestinal tract, the genitourinary tract and the secretory glands. McGhee, J. R. et al. Annals NY Acad. Sci.409:409 (1983). Mucosal antibodies act to limit colonization of the pathogen on mucosal surfaces, thus establishing a first line of defense against invasion. The production of mucosal antibodies can be initiated by local immunization of the secretory gland or tissue or by presentation of the antigen to either the gut-associated lymphoid tissues (GALT; Peyer's Patches) or the bronchial-associated lymphoid tissue (BALT). Cebra, J. J. et al. Cold Spring Harbor Syrup. Quant. Biol. 41:210 (1976); Bienenstock, J. M., Adv. Exp. Med. Biol. 107:53 (1978); Weisz-Carrington, P. et al., J. Immunol. 123:1705 (1979); McCaughan, G. et al., Internal Rev. Physiol. 28:131 (1983). Humoral immunity, on the other hand, results from the production of circulating antibodies in the serum (especially IgG and IgM), precipitating phagocytosis of invading pathogens, neutralization of viruses, or complement-mediated cytotoxicity against the pathogen. See, Hood et al. supra.
Others have noted that the induction of serum or mucosal antibody (humoral or cellular) responses to orally administered antigens, however, may be problematic. Generally, such oral administration requires relatively large quantities of antigen since the amount of the antigen that is actually absorbed and capable of eliciting an immune response is usually low. Thus, the amount of antigen required for oral administration generally far exceeds that required for parenteral administration. de Aizpurua and Russell-Jones, J. Exp. Med. 167:440-451 (1988). However, it has been found that the systemic and mucosal immune systems may be stimulated by feeding low doses of certain classes of proteins. In particular, this may be achieved with proteins which share the property of being able to bind specifically to various glycolipids and glycoproteins located on the surface of the cells on the mucosal membrane. Such proteins, called "mucosal immunogens" have been found to include viral antigens such as viral hemagglutinin. Moreover, dose-response experiments comparing oral with intramuscular administration revealed that oral presentation of mucosal immunogens was remarkably efficient in eliciting a serum antibody (humoral or cellular) response to the extent that the response elicited by oral presentation was only slightly lower than that elicited by intramuscular injection of the mucosal immunogen. de Aizupurua and Russell-Jones, supra.
The hypothesis proposed by these workers that such mucosal immunogens shared a common ability to bind glycosylated surface proteins on the mucosal membrane was at least partially confirmed by the inhibition of mucosal uptake of these mucosal immunogens by certain high levels of three specific sugars (galactose, lactose or sorbitol). Other sugars, fructose (the principal sugar found in many plant fruits) mannose and melibiose, did not inhibit mucosal immunogens from eliciting antibodies. de Aizupurua and Russell-Jones, supra. Others have found that certain sugars may, in fact, boost mucosal responses in the intestine. See, e.g., "Boosted Mucosal Immune Responsiveness in the Intestine by Actively Transported Hexose," S. Zhang and G. A. Castro, Gastroenterol, 103:1162-1166 (1992).
Recent advances in genetic engineering have provided the requisite tools to transform plants to contain foreign genes. Plants that contain the transgene in all cells can then be regenerated and can transfer the transgene to their offspring in a Mendelian fashion..sup.4 Both monocotyledonous and dicotyledonous plants have been stably transformed. For example, tobacco, potato and tomato plants are but a few of the dicotyledenous plants which have been transformed by cloning a gene which encodes the expression of 5-enolpyruvyl-shikimate-3-phosphate synthase..sup.5
Plant transformation and regeneration in dicotyledons by Agrobacterium tumefaciens (A. tumefaciens) is well documented. The application of the Agrobacterium tumefaciens system with the leaf disc transformation method.sup.6 permits efficient gene transfer, selection and regeneration.
Monocotyledons have also been found to be capable of genetic transformation by Agrobacterium tumefaciens as well as by other methods such as direct DNA uptake mediated by PEG (polyethylene glycol), or electroporation. Successful transfer of foreign genes into corn.sup.7 and rice,.sup.8,9 as well as wheat and sorghum protoplasts has been demonstrated. Rice plants have been regenerated from untransformed and transformed protoplasts. New methods such as microinjection and particle bombardment may offer simpler and even more efficient means of transformation and regeneration of monocotyledons..sup.10
Attempts to produce transgenic plants expressing bacterial antigens of Escherichia coli and of Streptococcus mutans have been made (Curtiss and Ihnen, WO 90/0248, 22 Mar. 1990). However, no transgenic plants have been constructed expressing viral antigens such as HBsAg. In particular, no such plants have been obtained which are capable of expressing viral antigens capable of eliciting an immune response as a mucosal immunogen. Moreover, no such plants have been obtained capable of producing particles which are antigenically and physically similar to the commercially available HBsAg viral antigens derived from human serum or recombinant yeast.
Thus, while prior approaches to obtaining less expensive and more accessible vaccines have been attempted, there remains a need to provide alternative sources of such vaccines for new antigens. For instance, while vaccines such as HBsAg have been produced using antigen particles derived from human serum and recombinant yeast cells, both sources require greater expense and provide lower accessibility to technically underdeveloped nations. Furthermore, while certain bacterial antigens may be expressed in transgenic plants, it is unknown whether antigens associated with human or animal viruses may be expressed in a form physically and antigenically similar to antigens used in commercial vaccines derived from human serum or recombinant yeasts. In particular, prior art approaches have failed to provide such commercially viable antigen from plants made to express transgenic hepatitis B viral antigens. Viral antigens, anti-viral vaccines and transgenic plants expressing the same as well as methods of making and using such compositions of matter are needed which provide inexpensive and highly accessible sources of such medicines.